In recent years, there is a demand for reducing the drive loss of automatic transmissions of automobiles in order to improve the fuel consumption of the automobiles. For the purpose of hydraulic sealing, a seal ring is attached to an automatic transmission. However, the friction loss of the seal ring leads to the drive loss of the automatic transmission. To reduce friction of the seal ring is therefore an important task. In addition, the capacity of an oil pump of the automatic transmission is a significant factor that causes the drive loss. There is therefore a demand for reducing the amount of oil leakage from the seal ring thereby reducing the capacity of the oil pump. To reduce the drive loss of the automatic transmission and improve the fuel consumption of the automobile, it is necessary that the seal ring should have low-friction characteristics and low-leakage characteristics.
FIG. 1 shows the basic structure of a hydraulic circuit using a seal ring. The seal ring 1 is attached to a shaft groove (ring groove) 4 formed on the outer peripheral surface of a shaft 2 on each of axially opposite sides of a hydraulic passage 3. Hydraulic oil supplied from the hydraulic passage 3 is received by a pressure-receiving side-surface 11 and an inner peripheral surface 12 of the seal ring. An outer peripheral surface 13 of the seal ring is in contact with the inner surface of a housing 5, and a contact side-surface 14 of the seal ring is in contact with a side surface of the shaft groove 4. The hydraulic pressure is thereby sealed. Generally, the shaft 2 is rotatable, and the housing 5 is stationary, or vice versa.
A method generally used to reduce the friction (friction loss) of a seal ring is to reduce a pressurizing load that presses the contact side-surface of the seal ring serving as a principal sliding surface against the ring groove. More specifically, a seal ring having a cross-sectional shape that allows the pressure of supplied oil to act between the contact side-surface of the seal ring and the ring groove is used to reduce the pressurizing load by the action of a cancelling load.
Patent Literature 1 discloses a method of reducing a pressurizing load by using a seal ring including side surfaces formed in a tapered shape in which an axial width decreases from an outer peripheral side toward an inner peripheral side whereby a cancelling load is generated between a ring groove and a side surface of the seal ring. The tapered shape formed by the side surfaces can significantly reduce the pressurizing load and is currently known as the shape of a seal ring that can minimize friction.
Patent Literature 2 discloses a seal ring that includes circumferentially spaced recessed sections (pockets) 6 formed at least on the inner peripheral side of the contact side-surface, and pillar sections 7 disposed between the recessed sections 6, as shown in FIG. 2A. As shown in FIGS. 2B and 2C, each of the recessed sections 6 includes a deepest inclined portion 51 formed such that the axial width (thickness) of the seal ring decreases in an inner circumferential direction, and converging portions 52 located on opposite circumferential sides of the deepest inclined portion 51 and converging toward the innermost peripheral points of adjacent pillar sections 7. In this configuration, when the seal ring rotates, the oil that fills the recessed sections 6 is squeezed along the inclined surfaces of the converging portions 52 to cause lift 60. In addition, hydraulic pressure acts on the recessed sections 6 on the contact side-surface to bring about a pressing load reduction effect (cancelling pressure 61). The friction is thus reduced. In the seal ring in Patent Literature 2, a side surface of the seal ring is in surface contact with the ring groove and slides thereon, as shown in FIG. 2D. Thus, no leakage passage is formed in a gap of the abutment joint of the seal ring, and low-leakage characteristics are thereby obtained.
In the seal ring in Patent Literature 1, the sliding contact between a side surface of the seal ring and the ring groove is line contact, and the circular sliding line is located on the gap of the abutment joint of the seal ring. As a result, the oil leaks from the gap of the abutment joint. Although the use of the recessed sections in Patent Literature 2 reduces the friction, the degree of reduction is lower than that in the seal ring in Patent Literature 1. There is therefore a demand for further reducing the friction.